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OSU: Yellowstone wolves influence elk; benefit trees, streams

August 27, 2007

The return of wolves to Yellowstone National Park has coincided with the growth of young aspen trees in the park’s northern range for the first time in 50 years, according to researchers from Oregon State University. The aspen trees are surviving the browsing of elk, which are a natural prey of wolves.

A study conducted by OSU forestry researchers supports a “trophic cascade” theory of ecological interdependence — extending to plants, animals, food chains and ecological zones — observed when key predators such as wolves are removed from an ecosystem. When predators are returned, recovery from damage is possible.

Wolves were reintroduced to Yellowstone National Park in 1995 after an absence of 70 years. Elk populations began a steady decline, cut in half over the past decade, according to the OSU researchers. The return of a natural predator may have altered the behavior of the remaining elk, which avoid browsing in certain areas where they feel most vulnerable to, or fear, wolves. This phenomenon has been called “the ecology of fear.”

The return of the wolf as predator and subsequent behavioral shift caused a significant reduction in elk browsing on young aspen shoots, allowing them to survive to heights where some are now above the animal browsing level, the researchers found.

“We’ve seen some recovery of willows and cottonwood, but this is the first time we can document significant aspen growth, a tree species in decline all over the West,” said William Ripple, a professor in the OSU College of Forestry.

The study found significant numbers of aspen, especially in streamside riparian zones, have grown from tiny shoots in the past decade to heights of more than seven feet &151; placing their crowns above the height easily browsed by elk and other animals. Tree growth in some stands has been particularly apparent just in the past four to five years.

The long-term decline (to the point of localized extinctions) of aspen and cottonwood trees in Yellowstone National Park dates to the extirpation of the last known wolf packs in the 1920s. Prior to the reintroduction of wolves, scientists found there were many small sprouting shoots of these tree species, and numbers of large trees 70 years old or more — but virtually nothing in between. High populations of grazing ungulates, primarily elk, had grazed on the small tree shoots at leisure and with little fear of attack.

But the ecological damage, the researchers said, went beyond trees. The loss of trees and shrubs opened the door to significant stream erosion. Beaver dams declined. Food webs broke down, and the chain of effects rippled (or cascaded) through birds, insects, fish and other plant and animal species. Thus, the term “trophic cascade.”

Aspen trees were a focus of concern for the OSU researchers. Unlike willows, aspen are more easily killed or suppressed by browsing and have been the slowest to recover. In some areas of the West, up to 90 percent of the aspen have disappeared, according to the OSU researchers.

“When I first looked at these degraded ecosystems in the mid-1990s in Yellowstone, I had doubts we would ever be able to bring the aspen back,” said Robert Beschta, a professor emeritus of forestry at OSU and co-author on the study. “There were so many elk, and the stream ecosystems were in such poor shape. The level of recovery we’re seeing is very encouraging.”

The OSU researchers determined there were two forces at work — lower populations of elk, and their changed behavior due to fear of wolves. It was difficult to determine exactly which force is the most significant.

Although the park’s elk populations declined after wolf reintroduction, elk populations now are actually higher than they were in the mid-1960s, when aspen trees were still in significant decline. The major change from that period of time is the presence of wolves. The effect of behavioral changes “may be equal to or even greater than” lower elk population levels in allowing tree survival, according to the researchers.

“In riparian zones, where wolves can most easily sneak up on elk, and gullies or other features make it more difficult for elk to escape, we’ve seen the most aspen recovery,” Ripple said. “We did not document nearly as much recovery in upland areas, at least so far, where elk apparently feel safer. But even there, aspen are growing better in areas with logs or debris that would make it more difficult for elk to move quickly.”

The element of fear, the OSU scientists said, is a behavoiral concept that is getting more attention in ecology. Predators such as wolves or cougars have the ability to “strike fear” into their prey and significantly change their behavior as a result.

The recovery of aspen, the researchers said, appears to have no link to climate or local terrain, since unbrowsed aspen in upland sites are growing just about as much as those in riparian zones. More details on this research may be found on OSU’s Trophic Cascades in Terrestrial Ecosystems Web site.

“The issue of aspen decline in the American West is huge, and their recovery will depend on local conditions and issues in many areas,” said Ripple said. “In northern Yellowstone, we finally have some good news to report.”

The findings of the OSU study were published in Biological Conservation.